The present invention relates to apparatus and methods for discriminating noise from valid signals in implantable medical devices, such as implantable pacemakers or defibrillators.
A pacemaker is a medical device that assists the heart in maintaining a desired rhythm. The heart is a pump that circulates life-sustaining blood throughout the body. There are four chambers in a human heart, right and left atria, and right and left ventricles. Blood returning from the body enters the right atrium. When full, the right atrium contracts and forces the blood through the tricuspid valve into the right ventricle. Once the blood has passed into the right ventricle, the right ventricle contracts and pushes the blood into the lungs. After passing through the lungs, where wastes are expelled and new oxygen is received, the blood returns to the left atrium. When full, the left atrium contracts and forces the blood through the bicuspid valve into the left ventricle. From the left ventricle, the ventricle contracts and forces the blood throughout the body.
The right and left atria contract simultaneously, as do the right and left ventricle. There is a delay (typically of from 50-200 milliseconds for most adult human hearts) between the time the atria contract and the ventricles contract. This delay allows sufficient time for the blood to move from the atria into the ventricles. For the atria to contract, the atrial muscle tissue must first depolarize. When depolarization of the muscle tissue occurs, there is manifest an electrical signal known as a P-wave that can be detected using electrocardiographic (ECG) devices. (For most purposes, depolarization of cardiac tissue can be considered to occur concurrent with the contraction of cardiac tissue.) Similarly, for the ventricles to contract, the ventricular muscle tissue must first depolarize, causing an electrical signal known as an R-wave (or sometimes a QRS complex) to be manifest, which R-wave can also be detected. The R-wave is much larger than the P-wave because the muscle tissue surrounding the ventricles is more massive than the muscle tissue surrounding the atria (as the ventricles have to pump the blood much farther than do the atria). Typically, the rate at which the heart beats is measured from R-wave to R-wave, as the R-wave, i.e., contraction of the ventricular tissue, is the easiest event to detect. If the heart is beating at 60 beats per minute, for example, there is one beat per second, or 1000 milliseconds between ventricular contractions.
A pacemaker provides electrical stimulation pulses to either the right atrium and/or right ventricle in order to stimulate the muscle tissue to cause a contraction. Demand pacemakers monitor the heart, through the same electrical leads through which the stimulation pulses are provided, in order to sense the occurrence of a P-wave and/or R-wave ("P/R wave"). If a P/R wave is sensed, then there is no need to deliver a stimulation pulse. In such an instance (when a P/R wave is sensed), the delivery of the stimulation pulse in a demand pacemaker is inhibited, thereby conserving the limited power of the pacemaker's battery, and further preventing irregular rhythms (contractions) of the heart muscle tissue that might otherwise result. Thus, a demand pacemaker provides stimulation pulses to the right atrium and/or right ventricle on demand, i.e., only when needed.
Similarly, automatic defibrillators provide a high energy stimulation pulse to cardiac tissue in an attempt to start contractions in a heart that has stopped. If the heart responds to such high energy defibrillation pulses and starts beating on its own, as manifest by, e.g., the pressure of R-waves at a more or less constant rhythm, the need for defibrillation pulses ceases. Thus, an automatic defibrillator also operates in a demand mode, providing defibrillation pulses only when needed.
The ability of a demand pacemaker or automatic defibrillator to properly perform its function of providing stimulation pulses on demand is critically dependent upon its ability to detect P/R waves. Unfortunately, many electrical signals may be present in a typical ECG signal (that signal sensed through the pacemaker or defibrillator leads) that do not represent valid P/R waves. Such signals are referred to as noise, and the pacemaker or defibrillator sensing circuits must utilize some means of reliably differentiating between noise and valid P/R waves.
One common technique that can be used to reduce noise in a pacemaker, or other implantable medical device, is a filter that limits the frequency of the signals that are allowed to pass through it. Because noise signals, especially white noise, tend to occur randomly over the entire frequency spectrum, the use of a filter thus significantly reduces the amount of noise present. However, as the P/R waves are themselves pulses, representing specific cardiac events (i.e., the depolarization of the atrial and/or ventricular muscle tissue), the frequency bandwidth of any filters that are used with P/R detection circuits must be quite broad. See, e.g., U.S. Pat. No. 4,686,988. Further, much of the noise present in an intracardiac ECG signal is not white noise, but is noise lying in the same frequency range as the P/R waves. Further, as "noise" can be any unwanted electrical signal, even signals associated with the heart (such as a T wave), it is not possible to limit noise to specific frequency ranges. Hence, while filtering aids the discrimination process to a certain extent, it is not effective at removing all noise from the signal.
Another common technique used to better discriminate noise from valid P/R waves is to employ a threshold detector or level detector. Such a circuit only passes signals therethrough having an amplitude that exceeds a prescribed threshold (reference) level. With such a circuit, low amplitude noise signals are rejected. Unfortunately, many noise signals within an intracardiac ECG signal are of a higher amplitude than is the P wave, as the P wave itself is a relatively low amplitude signal. Further, in some instances, short noise spikes may be present that approach or even exceed the amplitude of the R-wave. Thus, merely using a level detector does not remove all the noise signals from the ECG signal.
In response to the shortcomings of the filter and threshold detector for reliably discriminating valid P/R waves from noise, many prior art devices teach differentiating the intracardiac or other ECG signal. This is done because the P/R waves, particularly the R-wave, have a characteristic slope associated therewith that, if detected, can help identify a valid P/R wave from noise. U.S. Pat. No. 4,000,461, for example, teaches amplifying, filtering and differentiating the R-wave signal. The differentiated R-wave signal provides a signal proportional to the slope of the R-wave. This differentiated signal is then rectified, to look at both positive and negative slopes, and applied to a threshold detector, so that only signals having a slope above a set threshold level are acted upon. Finally, the signal is applied to a time discriminator to determine if the minimum slope is maintained for a prescribed period of time.
Similarly, in EPO 0 104 452 A1, valid R-waves are distinguished from noise by differentiating the ECG signal, and passing the resulting differentiated signal through a processing scheme that includes threshold detectors, rectifiers, and time discriminators.
While differentiating the ECG signal provides some basis for detecting a valid R-wave, it is difficult to use differentiation for successfully detecting a valid P-wave. This is because the P-wave, unlike the R-wave, is a relatively small signal that does not necessarily possess a sharp or characteristic slope. Further, the circuitry needed to perform the differentiation and other processing (e.g., rectification and time discrimination), represents additional circuitry that must be powered and housed within the limited power and space requirements of an implantable pacemaker or other implantable medical device. What is needed, therefore, is a noise discrimination technique that can readily distinguish both P-waves and/or R-waves from noise, and do so without requiring a large number of additional circuit components that may adversely impact the limited space and power requirements of an implantable pulse generator.
The present invention advantageously addresses the above-identified needs by providing a simple, yet effective, discrimination circuit that monitors the ECG signal directly, without differentiation or equivalent slope detection means, to determine whether a given ECG signal pulse is a valid ECG signal, i.e., a valid P-wave or R-wave, or noise.